A known circular knitting machine of this type (DE-Gbm 74 24 526) is provided with a rotatable blade, which is for the purpose of unraveling (cutting) the fabric tube, produced by the circular knitting machine, either between two stitch wales or in the region of what is known as a change point (changing zone), within which the threads forming the fabric tube are processed not to form stitches, but only to form float stitches. In order for the blade to be able to be installed in a simple manner in various types of machines and to be adjusted easily in terms of the position thereof relative to the fabric tube, said blade is connected to one end of a flexible shaft, the other end of which is provided with a drive wheel, which rolls on the underneath of a stationary carrier ring when the needle cylinder rotates. In this way, the flexible shaft, and with said shaft also the blade, are automatically set in rotation when the needle cylinder rotates.
In the known circular knitting machine, the drive wheel of the flexible shaft may also be formed as a friction wheel or as a toothed wheel. The construction as a friction wheel has the disadvantage that varying rotational speeds for the blade may result because of slipping or wear. As long as the fabric tube is to be cut in the region of float stitches, this is not a major problem. By contrast, if the fabric tube is to be cut along regions in which there is a conventional stitch formation, unclean or incorrect cutting edges may result, in particular at rotational speeds of the blade which are lower than corresponds to the speed of drawing-down the fabric tube. This is particularly undesirable in relation to circular knitting machines of which the draw-off and/or take-up rollers are provided with means for opening and stretching out the cut fabric tube so as to be able to draw off and/or take up a single-layered fabric with twice the width of the fabric tube (for example EP 0 456 576 B1, PCT WO 00/50678, DE 101 20 736 C1). By contrast, if the drive wheel of the flexible shaft is formed as a toothed wheel, then it is ensured that the blade always rotates at a pre-selected rotational speed. In this case, however, the stationary carrier ring would have to be provided with a circular toothed ring, and this involves considerable expense. Moreover, in both cases, it is difficult to access the drive wheel and it is thus difficult individually to adjust the rotational speed of the blade as a function of the speed at which the fabric tube is produced, or to replace the drive wheel.
In circular knitting machines of this last type, it is therefore conventional for the blade to be driven by a separate motor, which is in fact also arranged at an inaccessible location in the machine frame, but which can be set to different rotational speeds from the outside using electrical means. However, motors of this type increase the construction expenses and make exact positioning of the blade more difficult. In comparison therewith, the use of a flexible shaft would have advantages.